This invention relates to a process for producing cyanuric acid from urea as starting material. So far, the process for producing cyanuric acid by heating urea has been known in the art, but the process has the disadvantage that, since the reaction product is changed once from a solid phase to a liquid phase and again to the solid phase to complete the reaction, it may be agglutinated or solidified and adhere to the heat transfer surface to retard agitation and obstruct the process of heat transfer, with the result that the product quality is lowered. According to the former proposals to obviate such defect, the reaction is carried out by using a kneading device equipped with vanes for powerful agitation and scraping of the heat transfer surface or a rotary kiln wherein the agglomerated intermediate product is caused to flow during the reaction. Alternatively, the reaction is carried out by stirring a self-flowing particulate material obtained by spraying molten urea to the particulate reaction product, or by heating the material on a belt or a drum or above the molten metal. Some of these known methods are used on an industrial scale. While the above defect may be overcome to some extent by these known methods, they have some merits and demerits and moreover the operational procedure is troublesome and highly complex.